The present invention relates generally to porous, absorbent sheet material and more particularly is concerned with a new and improved electrode separator for electric storage cells employing an alkaline electrolyte.
Separators for isolating the positive and negative electrodes of alkaline cells typically take the form of a porous, relatively stiff, fibrous sheet material capable of withstanding the highly alkaline environment of the electrolyte used in these cells. Some of these separators have been made from predominantly cellulosic fibers, such as viscose rayon fibers, while others have been composed primarily of synthetic fibers, such as poly(vinyl alcohol) or polyolefin fibers. Still others have employed a combination of both cellulosic and synthetic fibers. See for example U.S. Pat. Nos. 3,915,750 and 4,746,586, the disclosures of which are incorporated herein by reference. Although both cellulosic and non-cellulosic fibers of the type mentioned are stable in the alkaline electrolyte solutions, each fiber type has its own particular advantages and disadvantages. For example, nonwoven sheet material made of predominantly regenerated cellulosic fibers exhibit the advantageous characteristics of high absorption and low cost. However, they also result in rather thick or bulky webs of high porosity and large pore size. The thickness of these webs limits their use to cells where this bulkiness can be tolerated. Additionally, they usually require a post formation treatment with a binder system such as a viscose binder and exhibit relatively high shrinkage within the plane of the sheet when exposed to the aqueous alkaline electrolyte. The non-cellulosic nonwoven fiber webs, such as those made from poly(vinyl alcohol) fibers, on the other hand exhibit almost no shrinkage in the alkaline electrolyte solution and advantageously use soluble poly(vinyl alcohol) as the binder during sheet formation. The poly(vinyl alcohol) sheet material is, unfortunately, of significantly higher cost and also is very thick, typically requiring calendering, wet pressing or the use of dryer felts to reduce the thickness thereof. It also requires the addition of a surfactant to improve its absorbency characteristics.
As indicated, attempts have been made to combine both cellulosic and synthetic fibers in order to achieve the benefits of both fibrous materials. However, the nonwoven webs using both types of fiber still have several disadvantages, the most prominent of which is the need for secondary processing such as calendering to reduce the thickness of the resultant material. Unfortunately, when the calendered separator material is immersed in the alkaline electrolyte solution, its thickness increases to such an extent, typically about 10% to 50%, that it regains most of its precalendered thickness, thereby creating a problem due to the limited space available within the cell.
It has now been found in accordance with the present invention that a modification in the fiber content of the separator sheet material will provide for the retention of the high absorbency characteristics of the cellulosic fibers, yet at the same time reduce the bulk or thickness of the sheet material and, importantly, reduce the planar shrinkage and size of the pores within the sheet material. This is achieved at substantially the same or lower basis weights without requiring secondary processing such as calendering or the like in order to reduce the thickness of the separator. The elimination of the calendering advantageously has the effect of limiting the thickness regain or swelling of the material in the alkaline electrolyte.
The adjustment in the fiber composition also provides a better means of controlling the pore size of the material while still using the advantageous features of the regenerated cellulosic fibers, namely, its absorbency, stability and cost. The reduced pore size achieved in accordance with the present invention significantly enhances the shelf life of the resultant cell.
The fiber control of the present invention also facilitates a reduction in the basis weight of the web material while maintaining the necessary barrier properties through pore size control and the necessary web strength through enhanced fiber-to-fiber contact. The separator volume within each cell is advantageously reduced thereby allowing increased cell performance.
Other advantages and features of the present invention will be in part obvious and in part pointed out more in detail hereinafter.
These and related features are achieved in accordance with the present invention by providing a new and improved separator of the type described in the form of a nonwoven fibrous web of water dispersible fibers that incorporates up to 100% by weight of fibers having a high cross-sectional aspect ratio, the fibers being uniformly distributed throughout the web material and anchored therein with the help of a suitable synthetic binder. The high cross-sectional aspect fibers include collapsible hollow fibers and ribbon fibers that have a significantly greater width than thickness in the dry fibrous web material. The high cross-sectional aspect fibers also appear to provide a synergistic reduction in shrinkage when combined with lower denier solid fibers of regular cross section.
A better understanding of the objects, advantages and relationships of the invention will be obtained from the following detailed description of separators possessing the features, properties and relation of elements exemplified herein. Included are illustrative embodiments that are indicative of the various ways in which the principles of the invention are employed.